1. Field of the Invention
The present invention relates to a method for fabricating an organic electroluminescent display that can improve the definition of an image at the edges of a pattern.
2. Description of the Related Art
An electroluminescent display is designed to realize an image by exciting an electroluminescent material disposed between electrodes by applying a voltage to the electrodes. Such an electroluminescent display is classified into an inorganic electroluminescent display and an organic electroluminescent display according to the electroluminescent material. The former is widely used for a backlight of a watch, while the latter is developed as a display as it advantages in that it can be driven with a low voltage and has a high responsiveness and polychrome.
Generally, such an organic electroluminescent display comprises a transparent substrate, an anode electrode formed on the substrate, an organic luminescent layer covering the anode electrode, and a cathode electrode formed on the organic luminescent layer.
The organic luminescent layer may have a variety of structures according to an electroluminescent material. For example, the organic luminescent layer may be formed of a hole transport layer, an luminescent layer, and an electron transport layer.
In the above described organic electroluminescent display, the organic luminescent layer is designed to realize red (R), green (G), and blue (B) colors so that it can be applied to a color display.
Such an organic luminescent layer is generally formed through a well-known vacuum evaporative deposition process or optical etching process. However, the vacuum evaporative deposition process has a limitation in reducing the physical gap between the patterns and it is difficult to form a minute pattern to tens of μm level which is required against the possible deformation of the mask. When the optical etching process is applied, although it is possible to form the minute pattern, the property of the luminescent material forming the organic luminescent layer may be deteriorated by the developing solution or the etching solution.
Therefore, in recent years, a thermal transferring method that is a kind of dry etching processes has been proposed to form the organic luminescent layer.
The thermal transferring method converts light emitted from a light source into thermal energy by which an image formation material is transferred to a substrate to form the organic luminescent layer. Therefore, to perform the thermal transferring method, a light source, a donor film and a substrate are required.
Describing the thermal transferring method more in detail, when light is emitted from a light source such as a laser unit to a donor film, the light is converted into thermal energy by a light absorption material of the donor film. The thermal energy allows the luminescent material of the donor film to be transferred to the surface of the substrate to form the organic luminescent layer.
That is, the organic luminescent layer is formed by scan laser beam to the door film disposed on the substrate. The focus of the laser beam is adjusted to a predetermined value, on the donor film disposed the substrate.
U.S. Pat. No. 5,521,035 discloses a method for fabricating a color filter for a liquid crystal display through a laser thermal transferring process.
In the patent, the color filter is fabricated by a laser induction thermal transferring process for transferring a color material from a donor film to a substrate such as a glass or a polymeric film. As a laser unit, an Nd:YAG laser system may be used. This will be described with reference to FIG. 1.
As shown in FIG. 1, the Nd:YAG laser is designed to form a Gaussian beam B1 shaped in a Gaussian functional distribution. When a diameter of the Gaussian beam B1 is set above 60 μm, the inclination of the energy distribution is reduced as it goes away from the center point O.
Accordingly, as shown in FIG. 2, when the organic luminescent layer is formed by the scanning operation of the Gaussian beam B1 having a predetermined diameter in an X-direction, since the beam intensity is low at the both edges 20 of the organic luminescent layer, the image quality at the both edges 20 is deteriorated when compared with the central portion.
When the energy of the laser beam is intensified to improve the image quality at the edges 20, since the energy is excessively increased at the central portion, the surface of the image pattern becomes irregular.
When the laser thermal transferring method is used to form the color filter, a transfer material (color material) is formed of a binder polymer and pigment inducing color change and dispersed in the binder polymer (acrylic acid resin or epoxy resin) at a ratio of 20-40%. The binder polymer simply functions for transmitting light. Therefore, to form a desire color pattern, a kind of binder polymer, a Tg (Glass Transmission Temperature) value or a molecular weight of the binder polymer should be properly modified.
Conventionally, the color material has a Tg value of about 60-120° C. and a molecular weight of about 1,500-5,000. The color layer formed by the color material has a thickness of about 1-2 μm.
When the laser thermal transferring method is used to form the organic electroluminescent display, the property (purity, Tg value, molecular weight and the like) of the transfer material (luminescent material) highly affects on the quality of the device. Therefore, when an inappropriate material is added, the quality of the device may suffer deathblow. Therefore, it is preferable to adjust the pattern quality by adjusting the laser transferring condition rather than modifying the property of the luminescent material.
Since the luminescent material used for the organic electroluminescent display has a molecular weight above 10,000 and a Tg value above 100° C., it is relatively difficult to perform the process for forming the organic luminescent layer when compared with the process for forming the color layer. The thickness of the organic luminescent layer which should be realized through the laser thermal transferring method should be 50-100 nm which is less than that of the color layer of the color filter.
Therefore, to form such a thin organic luminescent layer, scrupulous care in setting a laser beam transferring condition and an energy distribution is further required.